Overexpression and enzymatic characterization of Neisseria gonorrhoeae penicillin-binding protein 4.

The penicillin-binding proteins (PBPs) are ubiquitous bacterial enzymes involved in cell wall biosynthesis, and are the targets of the β-lactam antibiotics. The low molecular mass Neisseria gonorrhoeae PBP 4 (NG PBP 4) is the fourth PBP revealed in the gonococcal genome. NG PBP 4 was cloned, overexpressed, purified, and characterized for β-lactam binding, dd-carboxypeptidase activity, acyl-donor substrate specificity, transpeptidase activity, inhibition by a number of active site directed reagents, and pH profile. NG PBP 4 was efficiently acylated by penicillin (30 000 m⁻¹·s⁻¹). Against a set of five α- and ε-substituted l-Lys- d-Ala- d-Ala substrates, NG PBP 4 exhibited wide variation in specificity with a preference for N^ε-acylated substrates, suggesting a possible preference for crosslinked pentapeptide substrates in the cell wall. Substrates with an N^ε-Cbz group demonstrated pronounced substrate inhibition. NG PBP 4 showed 30-fold higher activity against the depsipeptide Lac-ester substrate than against the analogous peptide substrate, an indication that k₂ (acylation) is rate determining for carboxypeptidase activity. No transpeptidase activity was apparent in a model transpeptidase reaction. Among a number of active site-directed agents, N-chlorosuccinimide, elastinal, iodoacetamide, iodoacetic acid, and phenylglyoxal gave substantial inhibition, and methyl boronic acid gave modest inhibition. The pH profile for activity against Ac₂- l-Lys- d-Ala- d-Ala ( k_cat/ K_m) was bell-shaped, with p K_a values at 6.9 and 10.1. Comparison of the enzymatic properties of NG PBP 4 with other dd-carboxypeptidases highlights both similarities and differences within these enzymes, and suggests the possibility of common mechanistic roles for the two highly conserved active site lysines in Class A and C low molecular mass PBPs. [ABSTRACT FROM AUTHOR]